In JP-A-3-294750, conventionally, the following refrigeration method is disclosed. Higher-pressure refrigerant from the outlet of a condenser is divided to two portions. One of the divided-refrigerant portions is depressurized to an intermediate pressure by a first depressurizing device. This intermediate-pressure refrigerant exchanges heat with the other refrigerant portion to supercool the first. This supercooled refrigerant is depressurized by a second depressurizing device, and is introduced into an evaporator and vaporized therein. The intermediate-pressure refrigerant exchanges heat with the higher-pressure refrigerant, and is introduced into for compression into the compressor (gas-injection). The enthalpy difference (amount of heat-extraction) between the refrigerant at the inlet of an evaporator and the refrigerant at the outlet thereof is increased by supercooling the higher-pressure refrigerant, thereby improving the coefficient of performance (COP).
In the above-described prior device, heat exchange is performed only between the intermediate-pressure refrigerant and the higher-pressure refrigerant at the condenser outlet. Therefore, the higher-pressure refrigerant can be supercooled only to as high as the saturation temperature of the intermediate-pressure refrigerant. The present invention was developed in light of these drawbacks.